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Re: [Qemu-devel] Qemu varying performance

From: Edgar E. Iglesias
Subject: Re: [Qemu-devel] Qemu varying performance
Date: Wed, 28 Sep 2011 11:34:33 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Sep 28, 2011 at 02:25:43AM +0200, Torbjorn Granlund wrote:
> Running Debian's vmlinux-2.6.32-5-4kc-malta under qemu-system-mips works
> very well--the system is fast, and when it is idle the qemu-system-mips
> process on the host system consumes insignificant CPU.  (The same is
> true for qemu-system-mipsel, using the correesponding 'el' Debian
> kernel.)
> But running Debian's 64-bit kernel vmlinux-2.6.32-5-5kc-malta under
> qemu-system-mips64 consumes 100% on the host system, whether the guest
> is idle or busy.  (And for qemu-system-mips64el, the same is true for the
> corresponding 64-bit el kernel.)
> This cpu usage makes it somewhat inconvenient to let the qemu processes
> stay running.  Is there anything that can be done about the cpu usage?
> Is it a Linux problem (say, that the 64-bit kernel fails to invoke some
> clever system sleep instruction when in the idle loop), or is it a qemu
> problem?

Hi, It could be any of them or both. You'll have to dig a bit deeper and
see if WAIT insns are beeing issued and if QEMU is successfully decoding
them etc.

> Another issue I ran into what the sh4 qemu performance.  It is much
> worse than other qemu ports (except perhaps ppc64).  What is the reason
> behind that?  (The idle thing works well for sh4, though.)

IIRC, the SH has an MMU that is a bit problematic wrt performance in QEMU.


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